IL-7 is a non-redundant cytokine that plays a critical role in regulating the T-cell compartment of the immune system and is currently under investigation as a potential immune-reconstitution agent for various forms of immunodeficiency, including HIV infection. We previously demonstrated that IL-7 dramatically reduces levels of spontaneous ex vivo apoptosis in both CD4+ and CD8+ T cells derived from HIV-infected patients, while having no significant effect on cells taken from HIV-seronegative individuals. To investigate the effects of IL-7 in an in vivo model of HIV-1 infection, we performed a study of IL-7 treatment during the acute phase of SIV infection in a well-established nonhuman primate model of AIDS. Our study included 12 juvenile rhesus macaques divided into two groups: Group 1 (n=6) was infected with a pathogenic SIV isolate, SIVmac251, and received weekly injections of placebo;group 2 (n=6) was infected with SIV and concomitantly treated with fully glycosylated recombinant macaque IL-7 s.c., once per week for 7 weeks, at a dose of 50 ug/kg. They received no antiretroviral treatment. Multiple clinical, immunological and virological parameters were monitored in all study animals throughout the acute phase of SIV infection. Treatment with IL-7 was safe and did not increase the levels of SIV replication, while it positively affected the immunological profiles of SIV-infected macaques. Specifically, IL-7-treated animals had significantly higher absolute numbers of total circulating CD4+ T cells and CD8+ T cells when compared with untreated animals, and this effect persisted throughout the treatment period. Strikingly, treatment with IL-7 prevented the early depletion of peripheral blood CD4+ T cells that typically occurs during the acute phase of SIV infection. In particular, IL-7-treated animals, unlike untreated control animals, did not experience a depletion of naive and memory CD4+ T cells. This effect was associated with an increase in the intracellular expression of the anti-apoptotic protein Bcl-2, but not with a sustained proliferation of circulating CD4+ and CD8+ T cells, as measured by expression of the nuclear antigen Ki67. This finding suggests that apoptosis reduction was largely responsible for the reduced CD4+ T-cell depletion. This hypothesis was corroborated by studies in peripheral lymphoid tissue (axiliary lymph nodes) showing that IL-7 treatment reduced the levels of in vivo apoptosis of CD4+ and CD8+ T cells. IL-7-treated animals also developed earlier and stronger SIV Tat-specific T-cell responses associated with sustained increases in circulating CD8+ T cells. However, the effects of IL-7 were not sustained after cessation of treatment. Considerable attention has recently been focused on the gut-associated lymphoid tissue (GALT) as a major target for HIV infection, where critical events for pathogenesis take place. Specifically, the GALT is a primary anatomical site for virus replication, particularly during the early stages of HIV infection, leading to extensive depletion of CD4+ T cells. Since IL-7 plays an important role in peripheral T-cell homing, we investigated the ex vivo effects of exogenous IL-7 on the expression of a large panel of tissue-homing integrins and chemokine receptors. We observed that IL-7, in the absence of any concomitant stimulation, potently and selectively induces the expression of the principal gut-homing integrin, a4b7, in both CD4+ and CD8+ T cells. We found that this effect: i) is specific for T cells;ii) is rapidly induced upon IL-7 treatment;iii) requires supra-homeostatic concentrations of IL-7 (those typically reached under conditions of lymphopenia);iv) is uncoupled from the expression of classic markers of cellular activation;and v) is associated with the functional activation of the integrin, as indicated by an increased binding activity for its natural ligand, MAdCAM. Investigation of the molecular mechanisms of a4b7 induction by IL-7 revealed the involvement of both major signaling pathways linked to stimulation of the IL-7 receptor, i.e., the JAK/STAT and PI3K/Akt pathways. Induction of a4b7 by IL-7 was also confirmed in vivo, both in HIV-infected subjects and in SIV-infected macaques treated with IL-7. Of note, we found that induction of a4b7 by IL-7 occurs predominantly in phenotypically nave T cells, which concomitantly acquire a memory-like phenotype, as shown by upregulation of CD95 expression and secretion of TNF-a;upon stimulation with phorbol esters and ionomycin, despite an unaltered expression of CD45RA and CD45RO. This memory-like masquerade is similar to that previously documented in vivo in nave T cells of mice recovering from lymphopenia. Nave T cells were also induced to proliferate by IL-7, in the absence of any concomitant stimulation, albeit with delayed kinetics compared to a4b7 induction. These results are compatible with a new model of host response to lymphopenia whereby supra-homeostatic levels of IL-7 activate an unusual program of phenotypic modulation in nave T cells, characterized by the acquisition of a gut-homing and memory-like phenotype prior to the induction of cell cycling and proliferation. The role of intestinal T-cell homing in the reconstitution of the depleted T-cell pool in lymphopenic hosts remains to be defined. To formally demonstrate the physiological relevance of a4b7 induction by IL-7, we performed an in vivo study in which humanized NSG mice were injected with autologous T cells treated or not with IL-7. The results of these experiments clearly documented a preferential intestinal homing of IL-7-treated naive T cells, while no preferential homing to other tissues was detected. To investigate these phenomena and their relevance to AIDS in an in vivo model, we designed a new study in which 6 macaques chronically infected with either SIVmac251 or SIVsmE543 received a single injection of IL-7 (50 ug/kg, s.c.) and were sacrificed seven days later in order to specifically investigate the effects of IL-7 on T-cell homing and SIV replication in peripheral lymphoid tissues, particularly the GALT;three chronically SIV-infected animals received placebo and served as untreated controls. Detailed phenotypic analysis of circulating T cells documented a rapid upregulation of a4b7 in both CD4+ and CD8+ T cells. Comparison of pre-treatment and post-treatment intestinal tissues demonstrated that IL-7 administration resulted in increased numbers of infiltrating T cells within the GALT, associated with increased levels of SIV replication, predominantly in the Peyers patches. Enhanced SIV replication was also detected in lymph nodes. The increased levels of SIV replication in peripheral lymphoid tissues were mirrored by consistent increases in SIV plasma viremia. These in vivo data provided an initial validation of the hypothesis that the surge of endogenous IL-7 that occurs during the late stages of HIV infection may foster the terminal depletion of the CD4+ T-cell pool through the induction and activation of a4b7 leading to increased intestinal homing and HIV susceptibility.